Snail faunas in southern English calcareous woodlands: rich and uniform, but geographically differentiated

The snail faunas of 30 400m2 woodland sites distributed among the Chilterns, the western South Downs, the Cotswolds and the Wye Valley were sampled in a standardized way. Evidence shows that species inventories are nearly complete. All faunas show a high level of similarity to each other, on both presence and absence data, and on measures taking account of relative abundance. Sites from the same area cluster together in these analyses; this generally reflects differences in environmental conditions and history, which influence the occurrence and abundance of only a few species, rather than absolute geographical limitations on species. The Cotswolds have the richest faunas, due mainly to the wetland element at some sites. The Chilterns have the poorest; woodlands there are dry and mainly secondary. Nevertheless, the Chilterns hold three-quarters of all species found. Some of these sites are amongst the richest in N. Europe, and contain nearly all the regionally available fauna. These results are discussed in relation to earlier surveys in some of the same areas. Less intensive versions of the same sampling technique underestimate site richness and overestimate local heterogeneity. More extensive and repeated qualitative studies are far more effective for slugs, and provide good estimates of richness for areas of 25 ha or more. The results are also discussed in relation to the patterns seen in N. Europe and elsewhere.